1. Field of the Invention
This invention relates generally to the treatment of wastewater sludge and in particular, to a method of treating municipal, agricultural, food processing, and industrial liquid and semi-liquid sludge to form insoluble precipitates, increase the nutrient value of the residual precipitates, destroy pathogenic organisms, bind odors, and greatly improve the dewatering, drying, and handling characteristics.
2. Description of the Prior Art
The need to treat municipal wastewater sludge and to provide various methods to permit beneficial reuse of the sludge is well known in the art. Alkaline treatment is one such method and is an effective means for stabilizing sludge by increasing the pH of the sludge to 12 standard units (s.u.) or higher. Typically, the use of lime requires stabilization dosages of approximately 0.2 pounds of lime per pound of solids. If the sludge is also subjected to temperatures of 70.degree. C. or over for at least 30 minutes, the result is a complete disinfection, commonly referred to as pasteurization. Pasteurization results in the destruction of pathogens and indicator organisms making the sludge suitable for general plant nutrient supplementation and soil conditioning without a site specific application permit. The Environmental Protection Agency recognizes this type of process as a Process
That Further Reduces Pathogens (PFRP) in 40 C.F.R. .sctn.257 and as Class A in 40 C.F.R. .sctn.503 and allows unrestricted contact with biosolids so treated.
When calcium oxide (quicklime or CaO) or magnesium oxide (MgO) is added into sludge, the calcium oxide reacts exothermically with the free water of sludge to create calcium hydroxide (hydrated lime, Ca(OH).sub.2) plus heat. Calcium hydroxide may also continue to react with carbon dioxide in the atmosphere to form calcium carbonate. Although this carbonation reaction is exothermic, the typical heat available is modest due to the slow rate of the reaction.
Either method described above requires a high dosage of lime to generate a disinfecting heat by the exothermic reaction known as the heat of hydration. In order to achieve pasteurization temperatures of 70.degree. C., typical dosages are one pound of lime per pound of dry solids at a 20 percent initial solids concentration. The heat of hydration for lime is equivalent to approximately 491 BTUs per pound of lime. Additional chemical heats are available, such as carbon dioxide or the heat of carbonation, which release heat equivalent to approximately 780 BTUs per pound and may be used to reduce the lime requirement.
Numerous patents address the need for stabilization and sterilization. U.S. Pat. No. 4,270,279 issued to Roediger discloses a method of alkaline stabilization of dewatered sludge by the use of quicklime dust which results in the formation of pellets. The quicklime dust reacts exothermically with the surface of the pellets, resulting in a product that can be used as agricultural fertilizer.
U.S. Pat. Nos. 4,306,978and 4,997,572 issued to Wurtz disclose a method of producing sludge pellets using sludge stabilized by lime by the inclusion of calcium oxide or similar alkaline materials and combinations thereof with dewatered wastewater sludge cake in a compartmented reactor. The mixture reacts with carbon dioxide providing disinfection, stabilization, and pelletizing of the sludge. The Wurtz method requires that the dewatered sludge is provided in a sludge cake which is blended with calcium oxide to form sludge particles. The sludge particles are then mixed with calcium oxide to form calcium hydroxide raising the pH of the mixture to a range of 11 to 13. The sludge is then placed into a second compartment wherein the reaction of calcium oxide with free water continues to form calcium hydroxide and raises the pH of the mixture to a selected pH. The sludge is finally placed into a third vessel to react with induced calcium hydroxide and carbon dioxide gas forming calcium carbonate to provide the basis for pellet formation.
One problem with the prior art occurs during the addition of lime as a primary step for purposes of stabilization and/or disinfection. The receipt of wastewater is not a fixed or known variable hence causing the addition of lime to become an unknown variable. The result is an uninhibited and often costly addition of lime to the sludge until the desired pH stabilization occurs. Another problem is odor control which is not effectively addressed by the prior art. The control of odor directly affects the acceptability of a wastewater treatment process.
U.S. Pat. No. 5,429,750 issued to Steele, which is incorporated herein by reference, discloses a method to remove pathogens from dewatered sludge and to reduce vector attraction that includes raising the pH above 12 and subjecting the sludge to a low or near zero ambient pressure. The vacuum exposure enhances pathogen destruction and helps remove odiferous gases U.S. Pat. No. 5,385,673 issued to Fergen, which is incorporated herein by reference, discloses a method of treating dewatered biosolids by reducing the pH to below 5 s.u. for a predetermined period of time, followed by raising the pH above 12 s.u. Acid volatile compounds can be removed prior to alkaline conditioning. Base volatile compounds can then be removed. The separation of acid and base volatile compounds aids odor control. By selecting specific acid and base materials, the odor producing substances, such as ammonia, may be chemically bound, thereby reducing odor control requirements. Fergen at FIG. 1, col. 2, lines 46-52, and col. 4, lines 53-61.
What is unknown in the prior art, is a method of treating municipal, agricultural, food processing, and industrial liquid and semi-liquid sludge to form insoluble residual precipitates with increased nutrient value, bind odors, reduce vector attraction, and greatly improve the sludge's dewatering characteristics.